The present invention relates generally to satellites and satellite orbit raising methods, and more particularly, to apparatus and methods for raising the orbit of a satellite using electric propulsion.
Prior art orbit raising techniques are generally known to those skilled in the satellite art. For example, exemplary orbit raising techniques are described in U.S. Pat. No. 5,595,360 issued to Spitzer, entitled “Optimal Transfer Orbit Trajectory Using Electric Propulsion,” U.S. Pat. No. 6,116,543, issued to Koppel, entitled “Method and a System for Putting a Space Vehicle into Orbit, Using Thrusters of High Specific Impulse,” U.S. Pat. No. 6,543,723, issued to Oh, entitled “Electric Orbit Raising with Variable Thrust,” and U.S. Pat. No. 7,113,851, issues to Gelon et al., entitled “Practical Orbit Raising System and Method for Geosynchronous Satellites.”
The technique described in U.S. Pat. No. 5,595,360 attempts to first adjust the semi-major axis such that the orbit has a period equal to one sidereal day. This implies that the satellite's mean longitude does not change. Thereafter the eccentricity and inclination are adjusted until the desired GEO orbit is reached. This technique is suboptimal from a mass perspective, when compared to the invention disclosed herein. Furthermore, no strategy has been given as to how this method can be automated. Can this method be done effectively without a star tracker? What is the level of operator burden? The disclosed invention addresses these issues.
The technique described in U.S. Pat. No. 6,116,543 attempts to reach the desired orbit in at least a two stage process, when using electric propulsion. In the first stage, the apogee and perigee are adjusted by firing the thruster in a spiral trajectory, while minimizing the inclination. Thereafter in stage two, the apogee and perigee are adjusted independent of each other in a constant predetermined direction while also minimizing the inclination. This two step process is suboptimal when compared to the invention described herein. This technique also has operational challenges, with respect to spacecraft steering profile, power, telemetry monitoring, tracking, and commanding (TT&C) outages and autonomy, and may be of limited practical value.
The technique described in U.S. Pat. No. 6,543,723 requires variable thrust from Hall effect thrusters (SPTs). This patent addresses throttling the SPTs to operate at an optimum specific impulse levels. This patent details computing the optimum specific impulse for launching the satellite into orbit using chemical and electric propulsion devices. This patent addresses a slightly different aspect of Earth orbit raising (EOR) than does the invention described herein.
The technique described in U.S. Pat. No. 7,113,851 outlines a hybrid chemical and electric orbit raising method, and does not focus on the practicalities associated with carrying out such a task. It does not address how the electric orbit raising portion of the mission may be implemented with relative operational ease, nor does it address how such a scheme may be implemented without full 3-axis sensing, such as in the absence of a star tracker. It also does not address how momentum may be managed, without excessive use of chemical thrusters, when electric orbit raising is performed with only a single thruster. Furthermore no details pertaining to autonomy, such as those required during telemetry and command outages are provided. The invention disclosed herein addresses these practical details which are essential for a robust mission and its implementation.
It would be desirable to have apparatus and methods for raising the orbit of a satellite using electric propulsion that overcomes issues with previous techniques.